The fundamental mode Rayleigh wave generated by 27 earthquakes and recorded at 11 digital broadband seismic stations belonging to the Ocean Hemisphere Project network in the northwest Pacific region are analyzed to estimate the Rayleigh-wave group velocity for 160 paths at periods from 10 to 50 sec. The inverted group velocity results suggest a quasi-continental oceanic transitional structure of the northwest Pacific Ocean with 21 km as an average crustal thickness.

The Rayleigh-wave group velocity tomography results for short time periods (10–50 sec) showed a good correlation with known geological and tectonic features and were consistent with the tectonic complexities of the region. Slow-velocity anomalies at varying periods correspond to ridges, basins, trenches, and volcanic fronts of the northwest Pacific Ocean while the fast-velocity anomalies show the effects of plate interactions at relatively longer periods (30–50 sec). A triple junction formed by the subduction of the Phillipine Sea and Pacific plates beneath the Eurasian plate is imaged by a high-velocity zone in this study.

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